Precipitation hardenable nickel base superalloys of the gamma-gamma prime type are extensively used for gas turbine engine components. Many of these nickel base superalloys are difficult to fusion weld from the standpoint that cracking in the base metal heat-affected zone occurs during subsequent heat treatment to develop alloy mechanical properties (i.e. strain age cracking). One such precipitation hardenable nickel base superalloy is known as IN 939 having a nominal composition, in weight %, of 0.14% C, 22.58% Cr, 2.00% W, 19.00% Co, 1.90% Al, 3.75% Ti, 1.00% Nb, 1.40% Ta, and balance essentially Ni and strengthened by precipitation of gamma prime phase in the gamma phase matrix during subsequent heat treatment following welding. This alloy is considered to be only marginably weldable and to be highly susceptible to strain age cracking where objectionable cracking develops in the base metal heat-affected zone after welding during heat treatment to develop alloy mechanical properties.
A previously developed preweld heat treatment to avoid strain age cracking in IN 939 investment castings involved heating to 2120 degrees F. for 4 hours followed by slow cool at 1 degree F./minute or less to 1832 degrees F. and hold at that temperature for 6 hours followed by slow cool at 1 degree F. or less to below 1200 F. and finally gas fan cool to room temperature. However, the preweld heat treatment required 32 hours from start to completion, increasing the cost and complexity of manufacture of investment cast IN 939 components and necessitating long lead times and increased furnace capacity.
An object of the present invention is to provide a relatively short time preweld heat treatment that renders difficult or marginably weldable precipitation hardenable nickel base superalloys, such as the IN 939 nickel base superalloy, readily weldable without weld associated cracking during post-weld heat treatment.
Another object of the present invention is to provide a relatively short time preweld heat treatment that renders difficult or marginably weldable precipitation hardenable nickel base superalloys readily weldable without the need for alloy compositional modifications and without the need for changes to otherwise conventional fusion welding procedures.